Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
Multi-layered photoreceptors are known and described for example in U.S. Pat. Nos. 6,207,334, 6,197,464 and 6,242,144 (the disclosures of which are incorporated herein by reference). Such multi-layered photoreceptor structures are typically fabricated with a flexible or rigid substrate that is provided with an electrically conductive surface. A charge generating layer is then applied to the electrically conductive surface. A charge blocking layer may optionally be applied to the electrically conductive surface prior to the application of a charge generating layer. An adhesive layer may be utilized between the charge blocking layer and the charge generating layer. Usually the charge generation layer is applied onto the blocking layer and a charge transport layer is formed on the charge generation layer. This structure may have the charge generation layer on top of or below the charge transport layer.
Multilayer photoreceptors employing overcoatings of a crosslinked polyamide matrix are also known. For example, U.S. Pat. No. 5,702,854 to Schank discloses an electrophotographic imaging member that includes a supporting substrate coated with at least a charge generating layer, a charge transport layer and an overcoating layer, the overcoating layer comprising a dihydroxy arylamine dissolved or molecularly dispersed in a crosslinked polyamide matrix. The overcoating layer is formed by crosslinking a crosslinkable coating composition including a polyamide containing methoxy methyl groups attached to amide nitrogen atoms, a crosslinking catalyst and a dihydroxy amine, and heating the coating to crosslink the polyamide.
U.S. Pat. No. 5,681,679 discloses a flexible electrophotographic imaging member including a supporting substrate and a resilient combination of at least one photoconductive layer and an overcoating layer, the at least one photoconductive layer comprising a hole transporting arylamine siloxane polymer and the overcoating comprising a crosslinked polyamide doped with a dihydroxy amine.
U.S. Pat. No. 5,709,974 discloses an electrophotographic imaging member including a charge generating layer, a charge transport layer and an overcoating layer, the transport layer including a charge transporting aromatic diamine molecule in a polystyrene matrix and the overcoating layer including a hole transporting hydroxy arylamine compound having at least two hydroxy functional groups and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional groups of the hydroxy arylamine compound.
U.S. Pat. No. 5,368,967 discloses an electrophotographic imaging member comprising a substrate, a charge generating layer, a charge transport layer, and an overcoat layer comprising a small molecule hole transporting arylamine having at least two hydroxy functional groups, a hydroxy or multihydroxy triphenyl methane and a polyamide film forming binder capable of forming hydrogen bonds with the hydroxy functional groups the hydroxy arylamine and hydroxy or multihydroxy triphenyl methane. This overcoat layer may be fabricated using an alcohol solvent. This electrophotographic imaging member may be utilized in an electrophotographic imaging process. Specific materials include Elvamide polyamide and N,N′-diphenyl-N,N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine and bis-[2-methyl-4-(N-2-hydroxyethyl-N-ethyl-aminophenyl)]-phenylmethane.
U.S. Pat. No. 4,871,634 discloses an electrostatographic imaging member which contains at least one electrophotoconductive layer, the imaging member comprising a photogenerating material and a hydroxy arylamine compound represented by a certain formula. The hydroxy arylamine compound can be used in an overcoating with the hydroxy arylamine compound bonded to a resin capable of hydrogen bonding such as a polyamide possessing alcohol solubility.
U.S. Pat. No. 4,457,994 discloses a layered photosensitive member comprising a generator layer and a transport layer containing a diamine type molecule dispersed in a polymeric binder and an overcoat containing triphenyl methane molecules dispersed in a polymeric binder.
U.S. Pat. No. 4,599,286 discloses an electrophotographic imaging member comprising a charge generation layer and a charge transport layer, the transport layer comprising an aromatic amine charge transport molecule in a continuous polymeric binder phase and a chemical stabilizer selected from the group consisting of certain nitrone, isobenzofuran, hydroxyaromatic compounds and mixtures thereof.
U.S. Pat. No. 5,418,107 discloses a process for fabricating an electrophotographic imaging member including providing a substrate to be coated, forming a coating comprising photoconductive pigment particles having an average particle size of less than about 0.6 micrometers dispersed in a solution of a solvent comprising n-alkyl acetate having from 3 to 5 carbon atoms in the alkyl group and a film forming polymer consisting essentially of a film forming polymer having a polyvinyl butyral content between about 50 and about 75 mol percent, a polyvinyl alcohol content between about 12 and about 50 mol percent, and a polyvinyl acetate content is between about 0 to 15 mol percent, the photoconductive pigment particles including a mixture of at least two different phthalocyanine pigment particles free of vanadyl phthalocyanine pigment particles, drying the coating to remove substantially all of the alkyl acetate solvent to form a dried charge generation layer comprising between about 50 percent and about 90 percent by weight of the pigment particles based on the total weight of the dried charge generation layer, and forming a charge transport layer.
Single layer organic photoreceptors are also known. Such photoreceptors typically comprise a flexible or rigid substrate, which is provided with an electrically conductive surface. A relatively thick layer (over 30 microns) with combined charge transport molecules and charge generating pigments dispersed throughout the layer is applied to the substrate. The absorption and photogeneration in the single layer being concentrated near the top surface, and the associated absence of a pigment layer proximate to the substrate, renders the single layer organic photoreceptor superior to its multi-layer counterparts. For instance, the single layer photoreceptor has greater resolution because the image forming charge-packet does not have to traverse the entire thickness of the photoreceptor and therefore, does not spread. The single layer photoreceptor is also not susceptible to image plywood (an image defect seen in prints that is attributed to light interference which may be caused by light reflection off the substrate) since more light is absorbed by the photoreceptor, especially near the top surface. Therefore, the single layer photoreceptor does not require undercoat layers with light-scattering particles, substrate lathing and other substrate surface treatments to effect light scattering, or other plywood remedies currently employed on substrates. The single layer photoreceptor is also less susceptible to charge injection from the substrate to the pigment since there is no concentrated pigment layer next to the substrate and thus, does not require a charge (hole) blocking layer.
While more advantageous than multilayer photoreceptors, single layer photoreceptors do suffer from poor electron transport through the bulk of the relatively thick photoreceptor (30–40 microns). Further, single layer photoreceptors are positive charging. In order to comply with the predominantly negative charging development processes for most printers and copiers, there is a need for a negative charging photoreceptor having characteristics advantages of the single layer photoreceptor. There is also a need for a photoreceptor that has a lower unit manufacturing cost than current multi-layer designs, has improved wear rates, and allows for an overall thinner photoreceptor that enhances resolution.